bug#4623: 23.1.50; rmail changes encoding of characters on save

[Markus Rost]

> I think it goes a tad too far.  What I had in mind was way simpler:

I tried a bit by just adding something to after-save-hook.  But I ran
into problems with C-u g (rmail-get-new-mail from some file).

> record the encoding of the current message inside rmail-show-message,
> then restore it in after-save-hook.

I didn't try that.  Maybe that could work.

Actually, in the end my patch was meant to be simple and safe.  Its
changes can't be wrong (in the sense:  I proved the code is correct,
but didn't try it..., or just a few times).  It implements your
version of rmail-swap-buffers, which can't be wrong.  It removes
switching buffers in write-region-annotate-functions, which is wrong.
The buffer swapping with before/after-save-hook was at first meant as
a brutal but safe kludge, but on second thought it looks very natural
to me.